Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes than incandescent lamps. In addition, the LEDs require lower operating voltages and are more economical than fluorescent light sources.
Unfortunately, an LED produces light in a relatively narrow spectral band. Hence, to produce a light source that is perceived to have an arbitrary color, a compound light source having multiple LEDs is typically utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, green, and blue emitting LEDs. The ratio of the intensities of the various colors sets the color of the light as perceived by a human observer.
To provide the correct color over the lifetime of the light source, the intensities of the LEDs must be adjusted as the light source ages. The light generated by the individual LEDs changes over the lifetime of the LED. The rate of change varies with the type of LED and the particular production lot in which the LED is produced. Accordingly, the current through each LED must be adjusted to maintain the correct intensity as the LED ages.
One method for compensating for these shifts utilizes a feedback system that monitors the intensity of light generated by each LED and adjusts the drive current of each LED to maintain the light intensity at a predetermined value. If the light source consists of one LED of each color, the monitoring system can utilize a set of photodiodes and wavelength filters to measure the light generated by each LED. Unfortunately, the intensity of light that can be obtained from a single LED of each color is often too low for many applications. In such situations, light sources that utilize a plurality of LEDs of each color are often utilized. A single set of photodiodes cannot provide the individual intensities for the feedback loop in these systems. Accordingly, some form of monitoring system that measures the output from each LED separately is needed. In addition, each monitoring photodiode must be provided with a separate wavelength filter or be constructed from a material that is only sensitive in a narrow range of wavelengths. This increases the number of different parts that must be supplied to manufacture the light source, and hence, increases the cost and complexity of the light source.